With the rapidly increasing growth of direct reduction of iron throughout the world, there is an increasing shortage of iron oxide feed material in pelletized form, commonly called oxide pellets. Increasingly, there is an economic need to utilize crushed and sized natural lump ore as the oxide feed material for direct reduction. Most of the suitable natural lump ores have a much higher sulfur content than oxide pellets. Generally, oxide pellets have a very low sulfur content inasmuch as most of the sulfur present in the natural ore or concentrate from which the pellets are made is burned out during the firing of the pellets under oxidizing conditions.
When ores containing sulfur are used as the oxide feed material for direct reduction, much of the contained sulfur is liberated from the ore during the reduction process. Such liberated sulfur is present in the spent reducing gas as hydrogen sulfide gas (H.sub.2 S).
A highly efficient and commercially accepted direct reduction process is described in Beggs and Scarlett U.S. Pat. No. 3,748,120. In this process, spent reducing gas from a shaft type reduction furnace is mixed with hydrocarbon vapor and recycled through a catalytic reformer to produce fresh hot reducing gas. In the field of catalytic reforming of hydrocarbon vapor such as methane or natural gas, it is well known that the presence of H.sub.2 S in the reformer has an adverse effect on reforming. The reforming catalyst is deactivated or poisoned by H.sub.2 S, rendering the catalyst relatively ineffective. When an oxide feed material containing sulfur is used in the direct reduction process of U.S. Pat. No. 3,748,120, the H.sub.2 S present in the spent reducing gas is recycled to the catalytic reformer with resultant loss in reforming efficiency.
In the direct reduction of iron oxide to metallic iron, it is well known that the oxide is progressively reduced from hematite (Fe.sub.2 O.sub.3) to magnetite (Fe.sub.3 O.sub.4) to wustite (FeO) to metallic iron (Fe). In conventional gaseous reduction wherein reducing gas contains H.sub.2 and CO as reductants, and at conventional reduction temperatures in the range of about 1400.degree. F to 1770.degree. F (about 760.degree. to 930.degree. C), the reduction of hematite to magnetite to wustite occurs in about 30 to 45 minutes where oxide pellets or lump ore have a common particle size from about 1/4 inch to 1 inch. Metallic iron starts to form in the surface layer of the particle after this 30 to 45 minute period, and the complete reduction of the entire particle to metallic iron requires an additional 3 to 4 hours.
Extensive laboratory tests have been conducted in the direct reduction of sulfur-bearing iron oxide material using reducing gas having H.sub.2 and CO as reductants, and under conditions which simulate commercial scale reduction conditions. It has been determined that sulfur is liberated from the feed material during the first 30 to 45 minutes of the reduction cycle, after which time no more sulfur is liberated. The liberated sulfur is in the form of H.sub.2 S in the spent reducing gas and is readily measurable. When it was first observed that sulfur liberation ceases after the first 30 to 45 minutes of the reduction cycle, it was reasoned that metallic iron formed on the surface of the particles reacts with H.sub.2 S to form an iron-sulfur compound, and the presence of the metallic iron prevents further liberation of sulfur from the particles. To substantiate this theory, a special reduction test was conducted using a weak reducing gas which was thermodynamically incapable of reducing wustite to metallic iron. In other words, in this special reduction test, the final stage of reduction was wustite with no formation of metallic iron. In this special reduction test, it was determined that sulfur was liberated as H.sub.2 S continuously for a period of about 8 hours. In view of this observation, it is believed that the initial formation of metallic iron on the surface of the particles during normal direct reduction does, in fact, serve to prevent further sulfur liberation.